Breast cancer is the leading cause of death from cancer among women in the western world and the leading cause of death in general among persons 35 to 55 years of age. Imaging modalities for detection of breast lesions include X-ray mammography, sonography, thermography, computed tomography, angiography and magnetic resonance imaging (MRI).
The application of MRI to the imaging, localization (guidance to a lesion site) and treatment of breast lesions is copiously described in the literature. MRI literature references include MR Mammography, Kaiser, Werner A., Springer-Verlag, Berlin Heidelberg, 1993; Kaiser, Werner A., MRM promises earlier breast cancer diagnosis, Diagnostic Imaging, September, 1992; Svane, G., Stereotaxic Technique for Preoperative Marking of Non-Palpable Breast Lesions, Acta Radiolagical Diagnosis, 1983; Schnall, M. D., et al., A System for MR Guided Stereotactic Breast Biopsies and Interventions, Proceedings of the Twelfth Annual Scientific Meeting of the Society of Magnetic Resonance in Medicine, 1993, 1:163; Liu, Haiying, et al., Fat Suppression with an Optimized Adiabatic Excitation Pulse, Proceedings of the Twelfth Annual Scientific Meeting of the Society of Magnetic Resonance in Medicine, 1993, 3:1188; Hajek, Paul, et al., Localization Grid for MR-guided Biopsy, Radiology, 1987; Steger, A. C., et al., Interstitial Laser Hyperthermia, Br Medical Journal, 1989, 299; Castro, Dan, Metastatic Head and Neck Malignancy Treated Using MRI Guided Interstitial Laser Phototherapy, Laryngoscope 102, January, 1992; Ahlstom, K. Hakan, CT-guided Bone Biopsy, Radiology, 1993; Harms, S. E., MR Imaging of the Breast, JMRI, January/February, 1993; Heywang-Kobrunner, Sylvia, Nonmammographic Breast Imaging Techniques, Current Opinion in Radiology, 1992; Cosman, Eric, et al., Combined Use of a New Target-Centered Arc System, Proceedings of the Meeting of the American Society for Stereotactic and Functional Neurosurgery, Montreal 1987; Langlois, S. L., et al., Carbon Localization of Impalpable Mammographic Abnormalities, Australasian Radiology, August, 1991; Harms, S. E., et al., MR Imaging of the Breast with Rotating Delivery of Excitation Off Resonance, Radiology, 1993; Lagois, M. D., et al., The Concept and Implications of Multicentricity in Breast Carcinoma, Pathology Annual, Appleton-Century-Crofts, New York, 1981; Giorgi, C., et al., Three-dimensional Reconstruction of Neuroradiological Data, applied Neurophysiology, 1987; Heywang-Kobrunner, S. H. MRI of Breast Disease, Presented at the Twelfth Annual Scientific Meeting of the Society of Magnetic Imaging in Medicine, 1993; Liu, H., et al., Biplanar Gradient Coil Imaging (abstract), JMRI, 1993; Bown, S. G., Minimally Invasive Therapy in Breast Cancer (abstract), JMRI, 1993; and Derosier, C., MR and Stereotaxis, J. Neuroradiol, 1991. The disclosures of the above cited references are hereby incorporated by reference and liberally drawn from for this background section.
MRI can be realized because atoms with an odd number of protons or neutrons possess an intrinsic rotation or "spin" that, for clarity, may be likened to the spinning of a top. The atomic nucleus also carries an electric charge, and the combination of spin and charge leads to the generation of a magnetic field around the particle. The nucleus, then, represents a magnetic dipole whose axis is directed parallel to the axis of spin.
In the absence of an applied external magnetic field, the orientations of the proton spin axes are distributed statistically in space, so the magnetic dipoles cancel out in terms of their external effect. When a patient is placed into a magnetic field, the magnetic moments become oriented either parallel or antiparallel to the external field. Each state has a different energy level, the parallel alignment being the more favorable state in terms of energy. To alter these different energy states, the energy difference must either be added to or absorbed from the system from the outside. This can be accomplished by the application of an electromagnetic pulse at the magnetic resonance (MR) frequency or "Larmor frequency". In a magnetic field of 1 Tesla, for example, the Larmor frequency is 42 MHz.
The applied radio frequency pulse tilts the spin axis of the protons out of alignment by an angle that depends on the amplitude and duration of the transmitted electromagnetic pulse. A 90.degree. pulse is one that tilts the magnetization vector from the z axis to the xy plane, while a 180.degree. pulse causes a complete inversion of the magnetization vector.
After the excitation pulse has passed, relaxation commences as the nuclei return to their original states. This realignment process is characterized by a relaxation time T1 and corresponds to the motion of an electric charge in a magnetic field. As a result, the relaxation process causes the emission of an electromagnetic signal (the MR signal) from the nuclei that can be detected with special antennas (coils).
When the resonance frequency is applied to the sample as a 90.degree.pulse, the pulse not only tilts the magnetic moment 90.degree. but also tends to align the spin axes in the direction of the rf pulse. The angle of the spin axes is called the "phase". When the rf pulse ceases, the individual spins immediately begin to go out of phase. This "dephasing" process is called spin-spin relaxation and is characterized by a T2 relaxation time. The spin-lattice or T1 relaxation time describes the return of the magnetic moment to alignment with the external magnetic field. Both processes occur simultaneously in the same nucleus. Characteristic T1 values in biologic tissues range from 0.5 to 2 seconds and T2 values from 10 to 200 milliseconds.
By modifying the amplitude and duration of the applied rf pulses, an investigator can manipulate the alignment of the nuclear spins in varying degrees and for varying lengths of time. Accordingly, the MR signals generated by the tissue relaxation process vary greatly depending on the type of excitation pulses that are applied. The basic pulse sequences in clinical use include spin-echo, inversion recovery, gradient echo and fat suppression. Specialized pulse sequences under these general types include FLASH, FISP, RODEO and SNOMAN.
Image plane selection (slice selection) is accomplished by superimposing a linear gradient field upon a static magnetic field. Because the gradient field increases linearly in one direction, e.g., along the z axis, there is only one site at which the resonance or Larmor frequency condition is met. The bandwidth of an applied rf pulse and the steepness of the gradient determine the thickness of the tissue slice from which MR signals emanate. When two additional gradient fields are applied in the x and y directions, frequency or phase information can be assigned to different points within the selected plane.
A complete pulse sequence yields a raw-data image called a hologram. A 2-dimensional Fourier transform is applied to the raw data to construct the final image. Through the switching of magnetic gradients, sectional images can be constructed on a coronal, axial or sagittal plane or in any oblique orientation desired (coronal, axial and sagittal planes are respectively those dividing the frame into front and back portions, those dividing the frame into right and left portions and those dividing the frame into upper and lower portions).
Components of an MR unit include a primary magnet, shim coils whose current supply is computer controlled to produce the desired field homogeneity, gradient coils to generate linear gradient fields, an rf coil for transmitting the rf pulses and receiving the MR signals ( the signals may be received through the transmitting coil or a separate receiving coil), a computer for control of data acquisition, imaging parameters, and analysis and data storage media.
The rf excitation signal and the MR signal emitted by relaxing nuclear spins are respectively transmitted and received with rf coils types that include surface coils, whole-volume coils (in solenoid, saddle and birdcage configurations), partial-volume coils, intracavitary coils and coil arrays.
Breast coils are typically whole-volume solenoids used both for transmission and receiving. Such coils are especially suited for imaging frame regions that are perpendicular to the magnet aperture, e.g., breasts, fingers. They include square 4 pole resonators that can be inserted over the breast during imaging and Helmholtz pair resonators. Pairs of breast coils are often coupled to allow imaging of both breasts, e.g., see Model QBC-17 Phased Array Breast Coil, MRI Devices Corporation, 1900 Pewaukee Road, Waukesha, Wis.
The MR signal intensity varies exponentially with T1 and T2. Thus, a substance that alters the tissue relaxation times can be a potent image contrast enhancer. Gadolinium-diethylene triamine-pentaacetic acid (Gd-DTPA) is particularly suitable for producing contrast enhancement. Enhancement following injection seems to correlate with the vascularization of the lesion and the intense MR signal enhancement in carcinomas may be due to their increased vascular density.
Dynamic imaging involves repetitive imaging of the same slices before and after injection of Gd-DPTA. Dynamic, contrast-enhanced MR imaging has been found to be especially effective in differentiating benign from malignant lesions. MR signal increases (typically within the first minute after injection) can help differentiate carcinoma from benign breast lesions such as fibroadenoma, proliferative mastopathy, cysts, scars and mastopathies.
Numerous investigations and tests have demonstrated the high sensitivity (proportion of people having a disease that are so identified by a test) and specificity (proportion of people free of a disease that are so identified by a test) of MR imaging and its ability to detect even small cancers, e.g., 3-5 millimeters. However, successful imaging of breast lesions must be accompanied by effective guidance of medical instruments to the lesion site to facilitate diagnosis and treatment.
Accurate guidance is especially difficult in breasts because they lack rigid structure as, for example, in the cranium and can assume numerous configurations. FIGS. 1A, 1B and 1C are respectively front, top and side views of a breast 20 and illustrate how the location of a breast lesion 21 is typically described in relation to a coordinate system centered on the breast nipple 22. In these views, the lesion 21 exhibits cranial spacing 24, medial spacing 26 and posterior spacing 28 from the nipple 22. However, it is apparent that if the breast 20 were allowed to assume a configuration different from that of FIG. 1, these spacings would no longer accurately describe the lesion location. Thus, imaging and localization procedures are preferably completed without disturbing the breast position therebetween so that the imaging spacings used for localization are not corrupted.
Non-invasive localization or guidance techniques include measurement of the spacing between the lesion and the nipple and between the lesion and the overlying skin surface and transposition of the measurements to the breast surface where the calculated site is marked as a guide for a surgeon. Because of the considerations described above, non-invasive techniques generally permit only approximate guidance.
Invasive localization techniques often include apparatus for reducing breast movement and/or providing an MR visible coordinate. For example, perforated compression plates having rectangular apertures therein and an image visible coordinate system are described in Svane and Schnall in the above incorporated references. Gd-DTPA filled polyethylene tubes arranged in a grid and taped to an abdomen as a localization aid are described in Hajek in the above incorporated references.
Invasive treatment techniques include the insertion of a carbon trail leading to the lesion vicinity with a carbon trail injector as described in Svane and Langlois in the above incorporated references. The carbon trail serves as a marker to guide a surgeon to the lesion. Hook-wires are inserted to the lesion vicinity for the same purpose. They are typically removed during surgery. Introducing a fiber optic to the lesion vicinity for treatment with laser energy is described in Bown and Steger in the above incorporated references (interstitial laser photocoagulation or ILP in Bown; interstitial laser hyperthermia in Steger). In these treatment techniques, the laser fiber is typically passed through a thin needle to the lesion site.
Preferably, laser therapy is performed with the breast in a relaxed position to avoid forcing (as in compression techniques) a lesion proximate to the skin surface or urging separate lesions together thus losing spatial differentiation. In the first case, skin tissues may be destroyed and in the second case, healthy tissue between the lesions may unnecessarily be removed.
Other well known invasive procedures include the introduction of a needle for aspiration biopsy, a rotex screw biopsy needle within a cannula and a trocar within a cannula. In general, the goal of successful localization is the guidance of a medical instrument tip to the lesion site determined by imaging.
Because of the large magnetic fields involved in MR imaging, it is highly desirable that only nonferromagnetic materials be introduced within the magnetic fields. In addition, some materials can produce imaging artifacts (other sources of imaging artifacts include patient movement, heart movements, and chemical shifts due to resonance frequency difference of water and fat protons). Materials that do not exhibit nuclei relaxation will not appear on the MR image. On the other hand, if it is desired that a structure appear on the MR image, the material of that structure should exhibit nuclei relaxation.
Materials that do not cause imaging artifacts nor appear on the MR image shall hereinafter be called MR transparent while materials that are intended to appear on the MR image shall hereinafter be referred to as MR signal-producing. An example of an MR transparent material is polycarbonate. An example of an MR signal-producing material is Gd-DTPA contained in an MR transparent material.